2. Pacing during second half of lesions: During each of the 15 pre-specified lesions, pacing will be stopped at the halfway point (e.g. after 10 seconds for a 20-second lesion and after 15 seconds for a 30-second lesion). In the event that Wenckebach behavior is noted, pacing will be adjusted to a 550ms cycle length. In the event that Wenckebach behavior persists, the cycle length will be adjusted to 600ms. In the event that Wenckebach behavior persists, the pacing catheter will be moved to the right ventricle and pacing will be performed at a 500ms cycle length. If Wenckebach behavior still persists, the patient will be withdrawn from the study.